More and more people use their mobile devices and in particular their cellular phones (also referred to as user equipment (UE)) not only on the move, but also at home or in the office. As a result, an increasing number of local access networks are installed to provide these mobile devices with sufficient bandwidth and data transfer rates even in buildings or out of the service areas of macro cells. Typical macro cells are radio cells of the UMTS Terrestrial Radio Access Network (UTRAN) or the GSM/EDGE Radio Access Network (GERAN). The local access networks are pico- or femtocells, for instance. These local access networks cover small areas compared to the macro cells, such as in trains, aircrafts, ferries and building, like offices, private houses, shopping malls, train stations, airports and the like. An example of such a femtocell is described in US Patent Publication No. 2009/0092122 A1.
The above mentioned local access networks operate in parallel to the macro cells. As a result, there is usually a certain overlap between the local access network and the access network of the macro cells. In the case that the user equipment is located in the service area of a local access area network and does not have access to this local access area network, a data transfer between the radio network controller of the macro cell and the user equipment could potentially be disturbed by interference with the local access network (referred to as co-channel interference). In particular, if the carrier frequency for data transfers between the user equipment and the radio network controller is in the proximity of a frequency which is allowed for access to the local area network. An access to a femto network is not allowed, when the maximum amount of mobile devices in the femto network has already been reached, for instance.
This co-channel interference between the femto network and the macro network results in a “dead-zone” for “non-femto” user equipment in close proximity to the femto network and a “grey-zone” at the edge of femto coverage where macro quality can still be impaired. Normally in such cases the interfered macro base transceiver station (or Node B) will have at least one other frequency to serve the affected “non-femto” user equipment. The problem is that the radio network controller is not aware of the interference that “non-femto” user equipment may be experiencing near a disallowed femto network.
During the state transition from RRC idle to cell_DCH, or from cell_FACH to cell_DCH, the radio network controller has the option to redirect individual user equipment to a different UTRA cell by specifying the IE (information element) “Frequency info” in the RRC connection setup or Radio Bearer Reconfiguration message, respectively. However, at that point the radio network controller does not know if individual user equipment cannot or should not use that other frequency due to interference from the femto network. In the case that the target frequency of the RRC connection setup or the Radio Bearer Reconfiguration message is interfered by the local access network, the user equipment is not able to answer the radio network controller on the target frequency. Consequently, the radio network controller has to generate an additional RRC connection setup (and prior the user equipment has to transmit an additional connection request, respectively) or the radio network controller has to transmit a further Radio Bearer Reconfiguration message each concerning the clear frequency. This adds delay to the call setup time or to the Radio Bearer Reconfiguration time, each time.
Furthermore, there is a risk of a ping-pong scenario based on multiple inter-frequency handovers triggered by the radio network controller, wherein an inter-frequency handover from the clear frequency to the interfered frequency may be successful (for a short time), but shortly followed by an inter-frequency handover back to the clear frequency due to poor quality on the interfered frequency caused by interference with the femto network.